在数据链路层中，帧的大小各式各样，怎样把不同的帧区分开来？

字节填充就是用来干这个事情的。它的实现原理很简单，就是在每个帧的开头和结尾加上一个8位的标识F，用这个标识来区分每一个帧。

那么问题来了，如果帧本身的内容就包含这个标识F呢？也不复杂，可以类比正则表达式，用转义字符修饰普通的字符F就可以了，这里我们同样用一个8位标识E作为转义字符，你可能又会问，那如果帧本身内容也包含这个转义字符E呢？我们在转义字符前面再加上一个转义字符即可。

举个例子：

1
2
3
4
5
6
7
8
9
发送端将要发送的原始内容 : 
DDEDFFDE
字符填充之后的内存变为 : 
FDDEEDEFEFDEEF

接收端接收到的内容 :
FDDEEDEFEFDEEF
还原成原本的内容 : 
DDEDFFDE

用Java实现客户端与服务端
  • 客户端
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
// Java Code for Byte_Stuffing Sender
package byte_stuffing;
import java.io.*;
import java.util.*;
import java.net.*;

public class Byte_Stuffing_Client {
	public static void main(String args[]) throws IOException
	{
		InetAddress ip = InetAddress.getLocalHost();
		int port = 45678;
		Scanner sc = new Scanner(System.in);

		// Opens a socket for connection
		Socket s = new Socket(ip, port);

		// Declaring I/O Streams
		DataInputStream dis = new DataInputStream(s.getInputStream());
		DataOutputStream dos = new DataOutputStream(s.getOutputStream());

		while (true) {
			System.out.println("Enter the Message to be Sent : ");
			String data = sc.nextLine();
			String res = new String();

			// Data in each frame is stuffed by 'F' at beginning and end
			data = 'F' + data + 'F';
			for (int i = 0; i < data.length(); i++) {

				// Stuff with 'E' if 'F' is found in the data to be sent
				if (data.charAt(i) == 'F' && i != 0 && i != (data.length() - 1))
					res = res + 'E' + data.charAt(i);

				// Stuff with 'E' if 'E' is found in the data to be sent
				else if (data.charAt(i) == 'E')
					res = res + 'E' + data.charAt(i);
				else
					res = res + data.charAt(i);
			}

			System.out.println("The data being sent (with byte stuffed) is : " + res);

			// Send the data to the receiver
			dos.writeUTF(res);
			System.out.println("Seding Message....");
			if (dis.readUTF().equals("success"))
				System.out.println("Thanks for the Feedback Server!!");

			// End Messaging
			dos.writeUTF("bye");
			break;
		}

		// Close all connections
		s.close();
		dis.close();
		dos.close();
	}
}


  • 服务端
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
// Java code for Byte_Stuffing Receiver
package byte_stuffing;
import java.io.*;
import java.net.*;

public class Byte_Stuffing {
	public static void main(String[] args) throws IOException
	{
		// Opens a socket for connection
		ServerSocket servsock = new ServerSocket(45678);

		// Used to block until a client connects to the server
		Socket socket = servsock.accept();

		// Declaring I/O Streams
		DataInputStream dis = new DataInputStream(socket.getInputStream());
		DataOutputStream dos = new DataOutputStream(socket.getOutputStream());

		while (true) {
			String out = new String();
			// Used to read the data sent by client
			String res = dis.readUTF();
			System.out.println("Message Received...Successfully!!!");
			System.out.println("The Stuffed Message is : " + res);

			for (int i = 1; i < res.length() - 1; i++) {

			// If data contains a 'D' or 'F' do not unstuff it
				if (res.charAt(i) == 'D' || res.charAt(i) == 'F')
					out = out + res.charAt(i);

				// If data contains 'E' followed by 'E', de-stuff the former 'E'
				else if (res.charAt(i) == 'E' && res.charAt(i + 1) == 'E') {
					out = out + 'E';
					i++;
				}
			}
			System.out.println("The Destuffed Message is : " + out);
			dos.writeUTF("success");
			String ch = dis.readUTF();
			if (ch.equals("bye")) {
				System.out.println("Messaging is over.....EXITING");
				break;
			}
		}

		// Closing all connections
		socket.close();
		dis.close();
		dos.close();
	}
}

这里要注意一点，服务端应该先于客户端启动，因为服务端要先开启一个socket客户端才能进行连接


本文作者：oldmee
本文链接：https://oldmee.github.io/2015/08/23/Byte-Stuffing-using-Java/
版权声明：本博客所有文章除特别声明外，均采用 CC BY-NC-SA 4.0 许可协议。